Carbon nanotube gas sensor conductance model, sensing mechanism analysis, and applications in flexible sensors and wireless sensors.

摘要

In this dissertation, the electrical property dependency of carbon nanotubes (CNTs) upon the humidity and chemical gas concentration is investigated. The electrical response of single-walled carbon nanotube (SWNT) networks to different humidity levels and gas molecules of ammonia (NH3) and nitrogen dioxide (NO2) at different concentrations was characterized by a sensor test system. In order to exclude the effect from contact resistance, the sheet resistance of SWNT networks was measured by transfer length method. The gas molecules concentration dependence of electrical property was analyzed, and two electrical models were proposed based on carrier transportation and adsorption isotherm respectively for different gas molecules. The electrical properties of SWNT networks estimated by the models were compared with the experimental data. The results show the models agree well with the experimental data. The primary objective of the sensor model is to understand the relationship between conductance of CNT networks and gas concentration. With this understanding, the model offers a precise measurement of the gas concentration based upon the electrical property of SWNT sensor.;In addition, the mechanism of gas molecule adsorption on CNT networks is analyzed based on the conductance models. It is found that the conductance change of SWNT networks, induced by different humidity levels, is dominated by the thermal activation carrier hopping over the barriers between SWNTs. The average separation between the SWNTs increases linearly with the humidity levels. By contrast, when exposed to different NH3 and NO2, the conductance change is primarily determined by the charge transfer between gas molecules and CNTs. It shows that chemical molecules adsorption on the surface wall of SWNT causes the charge transfer.;Furthermore, we investigated printed flexible electronics based on SWNTs and printable SWNT-based Frequency modulation (FM) passive wireless sensor tag on a flexible substrate with enhanced sensitivity.;In this dissertation, we report a high sensitive flexible SWNT chemical gas sensor, capable of being easily fabricated. The sensor also shows good performance in linearity and flexibility. The flexible printed SWNT sensor with high sensitivity, low LOD and flexibility provides a promising solution to low-cost flexible sensor with high performance for mass production.;Last, we developed a printable SWNT-based passive FM wireless sensor tag on a flexible substrate for ammonia detection. The FM wireless sensor tag shows an enhanced sensitivity and also exhibits a high linearity between the frequency shift and the logarithm of the chemical gas concentration. The linear response allows one to precisely predict the NH3 concentration by measuring the frequency shift of the FM wireless sensor tag. The experimental demonstration of the passive wireless sensor tag and accurate measurement of the NH3 concentration levels indicate that the FM modulated passive wireless sensor tag is promising for power-less standalone low-level NH3 sensing and monitoring.